GENEVA (AP) The head of the World Health Organization called Wednesday for a moratorium on administering booster shots of COVID-19 vaccines as a way to help ensure that doses are available in countries where few people have received their first shots.

WHO Director-General Tedros Adhanom Ghebreyesus made the appeal mostly to wealthier countries that have far outpaced the developing world in numbers of vaccinations. He said richer countries have administered about 100 doses of coronavirus vaccines for every 100 people on average, while low-income countries hampered by short supplies have provided only about 1.5 doses per 100 people.

WHO officials say the science is unproven about whether giving booster shots to people who have already received two vaccine doses is effective in preventing the spread of the coronavirus.

The U.N. health agency has repeatedly called for rich countries to do more to help improve access to vaccines in the developing world. It has argued that no one is safe until everyone is safe because the longer and more widely the coronavirus circulates, the greater the chance that new variants could emerge and prolong a global crisis in fighting the pandemic.

The agency has no power to require countries to act, and many in the past have ignored its appeals on issues like donating vaccines, limiting cross-border travel and taking steps to boost production of vaccines in developing countries.

Tedros pointed to a WHO target he had announced in May seeking to ensure that 10% of the populations in all countries receive vaccines against the coronavirus.

Accordingly, WHO is calling for a moratorium on boosters until at least the end of September to enable at least 10% of the population of every country to be vaccinated, he told a news conference.

To help take the heat out of the pandemic, WHO has been focusing on getting vaccines to older adults, health care workers and other target populations in many countries before booster shot campaigns are carried out.

Dr. Bruce Aylward, a special adviser to Tedros, said the moratorium was about an appeal to countries considering booster doses to put a hold on such policies until and unless we get the rest of the world caught up in the fight against the pandemic.

As weve seen from the emergence of variant after variant, we cannot get out of it unless the whole world gets out of it together. And with the huge disparity in vaccination coverage, were simply not going to be able to achieve that, Aylward said.

Israel, France, Germany and many Middle Eastern countries have already started administering boosters, and other nations, including the United States and Britain, are considering plans to do so in the wake of the emergence of the highly transmissible delta variant.

Dr. Katherine OBrien, WHOs vaccines chief, noted that a very limited number of countries were giving booster doses though a larger number were contemplating it.

The evidence is evolving. Its moving. We dont have a full set of evidence around whether this is needed or not, OBrien said, adding that the main message was that we need instead to focus on those people who are most vulnerable.

WHO officials reiterated their call for global solidarity to help battle the coronavirus pandemic and appealed to wealthy countries and corporations to help.

We need everyones cooperation, especially the handful of countries and companies that control the global supply of vaccines, Tedros said, appealing in particular to the influential Group of 20 large economies. The G-20 has a vital leadership role to play as the countries that are the biggest producers, the biggest consumers and the biggest donors of COVID-19 vaccines.

He urged the G-20, which currently is chaired by Italy, to make concrete commitments to support global vaccination targets.

We call on everyone with influence Olympic athletes, investors, business leaders, faith leaders and every individual in their own family and community to support our call for a moratorium on booster shots until at least the end of September, Tedros said.